/*
 * @filename SmallCharMatcher.java
 * @author barry
 * @version 0.0.1
 * @date 2020年3月4日
 */
package com.bnzj.core.persistence.query.conversion;

import java.util.BitSet;

import com.bnzj.core.persistence.query.conversion.CharMatcher.NamedFastMatcher;

/**
 * @author barry
 * @date 2020-03-04
 */
final class SmallCharMatcher extends NamedFastMatcher {
    static final int MAX_SIZE = 1023;
    private final char[] table;
    private final boolean containsZero;
    private final long filter;

    private SmallCharMatcher(char[] table, long filter, boolean containsZero, String description) {
      super(description);
      this.table = table;
      this.filter = filter;
      this.containsZero = containsZero;
    }

    private static final int C1 = 0xcc9e2d51;
    private static final int C2 = 0x1b873593;

    /*
     * This method was rewritten in Java from an intermediate step of the Murmur hash function in
     * http://code.google.com/p/smhasher/source/browse/trunk/MurmurHash3.cpp, which contained the
     * following header:
     *
     * MurmurHash3 was written by Austin Appleby, and is placed in the public domain. The author
     * hereby disclaims copyright to this source code.
     */
    static int smear(int hashCode) {
      return C2 * Integer.rotateLeft(hashCode * C1, 15);
    }

    private boolean checkFilter(int c) {
      return 1 == (1 & (filter >> c));
    }

    // This is all essentially copied from ImmutableSet, but we have to duplicate because
    // of dependencies.

    // Represents how tightly we can pack things, as a maximum.
    private static final double DESIRED_LOAD_FACTOR = 0.5;

    /**
     * Returns an array size suitable for the backing array of a hash table that uses open addressing
     * with linear probing in its implementation. The returned size is the smallest power of two that
     * can hold setSize elements with the desired load factor.
     */
    static int chooseTableSize(int setSize) {
      if (setSize == 1) {
        return 2;
      }
      // Correct the size for open addressing to match desired load factor.
      // Round up to the next highest power of 2.
      int tableSize = Integer.highestOneBit(setSize - 1) << 1;
      while (tableSize * DESIRED_LOAD_FACTOR < setSize) {
        tableSize <<= 1;
      }
      return tableSize;
    }

    static CharMatcher from(BitSet chars, String description) {
      // Compute the filter.
      long filter = 0;
      int size = chars.cardinality();
      boolean containsZero = chars.get(0);
      // Compute the hash table.
      char[] table = new char[chooseTableSize(size)];
      int mask = table.length - 1;
      for (int c = chars.nextSetBit(0); c != -1; c = chars.nextSetBit(c + 1)) {
        // Compute the filter at the same time.
        filter |= 1L << c;
        int index = smear(c) & mask;
        while (true) {
          // Check for empty.
          if (table[index] == 0) {
            table[index] = (char) c;
            break;
          }
          // Linear probing.
          index = (index + 1) & mask;
        }
      }
      return new SmallCharMatcher(table, filter, containsZero, description);
    }

    @Override
    public boolean matches(char c) {
      if (c == 0) {
        return containsZero;
      }
      if (!checkFilter(c)) {
        return false;
      }
      int mask = table.length - 1;
      int startingIndex = smear(c) & mask;
      int index = startingIndex;
      do {
        if (table[index] == 0) { // Check for empty.
          return false;
        } else if (table[index] == c) { // Check for match.
          return true;
        } else { // Linear probing.
          index = (index + 1) & mask;
        }
        // Check to see if we wrapped around the whole table.
      } while (index != startingIndex);
      return false;
    }

    @Override
    void setBits(BitSet table) {
      if (containsZero) {
        table.set(0);
      }
      for (char c : this.table) {
        if (c != 0) {
          table.set(c);
        }
      }
    }
  }
